The present invention relates to an absolute angle calculation apparatus which calculates an absolute angular position with respect to a reference angular position with a rotary encoder incorporated, for example, in a rotary laser apparatus or the like.
As illustrated in FIG. 1, one of known surveying instruments such as a rotary laser apparatus 1 is configured to detect an absolute angular position θ of a target 2 with respect to a reference angular position Q of the rotary laser apparatus 1 by rotating a rotating prism (not illustrated), for example, at three revolutions per second (180 rpm), projecting a fan beam BP onto the target 2, and receiving a reflected light beam BP′ from the target 2.
The rotary laser apparatus 1 subsequently projects a laser beam to the target 2, for example, at the detected absolute angular position θ, and receives the reflected light beam BP′ from the target 2 to measure a distance between the rotary laser apparatus 1 and target 2.
The rotary laser apparatus 1 of this type internally includes a rotary encoder 3 illustrated in FIGS. 2A and 2B. The rotary encoder 3 mainly comprises a rotating plate 4, a light emitting unit 5, and a light receiving unit 6.
A large number of transmissive slits 7 are formed in a circumferential direction in a peripheral part of the rotating plate 4. The large number of transmissive slits 7 include, for example, first to third patterns P1 to P3 which are spatially modulated as illustrated in FIG. 2C.
For example, Japanese Patent No. 3168451 discloses that the patterns P1 to P3 are formed at a constant pitch width P. A pattern width of the pattern 1 is fixed, while the widths of the patterns P2 and P3 are modulated according to a predetermined rule.
The light emitting unit 5 and light receiving unit 6 are arranged to face a slit-formed region of the rotary plate 4 and to face each other with the rotary plate 4 interposed therebetween. The light emitting unit 5 comprises, for example, a light emitting element 5a and a collimator lens 5b, and the light receiving unit 6 comprises, for example, a linear sensor. The light receiving unit 6 receives a light beam from the light emitting unit 5 through the transmissive slits 7 and converts the light beam to a photoelectrically-converted signal string. Then, the photoelectrically-converted signal string is inputted to a calculation circuit (not illustrated).
The calculation circuit calculates the absolute angular position θ with respect to the reference angular position Q on the basis of a spatially modulated component contained in the photoelectrically-converted signal string in the light receiving unit 6.
Meanwhile, it usually takes about 100 milliseconds (ms) for the calculation circuit of this type to calculate the absolute angular position θ. On the other hand, the rotary encoder 3 of this type rotates, for example, about 108° (degrees) for 100 ms. Accordingly, there is a possibility that when a measurement is performed every 100 ms, the absolute angular position θ of the target 2 with respect to the reference angular position Q may not be precisely calculated.
Moreover, there is another possibility that in a case of a simultaneous measurement of two or more targets, if the two or more targets are within an angular position range of less than 108°, the absolute angular position of at least any one of the targets may not be measured.